Charge Transfer Induces Giant Nonlinear Optical Response Enhancement in γ-Graphdiyne Functionalized with Superalkalis Li ₂ X (X = F, Cl, Br) and NY ₄ (Y = H, Li)

This study addresses the critical need for enhanced nonlinear optical (NLO) materials by strategically doping γ-graphdiyne (GDY), a two-dimensional carbon allotrope with inherent NLO sensitivity, with superalkalis. Leveraging the ultra-low ionization potentials of superalkalis, we introduce novel dopants beyond conventional metal-oxide clusters: halogen-based Li 2 X (X = F, Cl, Br) and nitrogen-based NY 4 (Y = H, Li). Employing density functional theory (DFT) at the ωB97XD/6-31G(d,p) level, ground-state geometries, interaction energies, and charge transfer mechanisms were systematically evaluated. The sobEDAw analysis explicitly resolves the bonding essence between superalkalis and GDY as a closed-shell non-covalent donor-acceptor interaction, dominated by electrostatic contributions (51~56% of total attractive energy) with supplementary dispersion (33~37%) and induction (11~14%) components. Time-dependent DFT (TD-DFT) with the CAM-B3LYP functional quantified dynamic NLO properties, focusing on the first hyperpolarizability. The Polarizable Continuum Model (PCM) calculations assessed solvent effects. A comparative study with graphene and graphyne was conducted to establish the structural advantage of GDY. Results reveal stable adducts governed by non-covalent interactions, with vertical distances and interaction energies dependent on dopant polarizability and size. Charge transfer from superalkalis to GDY significantly reduces the HOMO-LUMO gap (from 7.07 eV in pristine GDY to 3.5 eV in NH4@GDY), and dramatically enhances first hyperpolarizability up to 5.28 × 10 4 a.u. for NH 4 @GDY, versus 0.07 a.u. in pure GDY. The second hyperpolarizability (γ) is also significantly enhanced, reaching 1.42 × 10 8 a.u. for NH4@GDY. The two-level model attributes this enhancement to minimized excitation energies and optimized oscillator strengths. Model size convergence tests validate the reliability of the cluster model. This work establishes halogen/nitrogen superalkalis as potent dopants for GDY-based NLO materials, providing foundational insights for next-generation photonic devices.